If the 10 has a user equalizer, make sure the human voice range isn't turned down. Different sources peg that range between 300Hz and 3500Hz. If you've got a slider in the middle of that range, slide it up.

Try a different pair of headphones. Certain ones after a long time of use can't play certain ranges, goes into MONO rather than stereo and many other flaws.
Also try finding songs with a sample rate of 320khz it's the best to get as far as I know, 132 or something like that is low but still good enough quality. Anything lower than that I've found caused issues with my T-10 to sorta lock up on alot of things.
Also try disabling the DNS if it's enabled. Further more, upgrading firmware will do the trick.

But again: I think it's just your headphones are faulty my brother's IPOD (Curse that POS) headphones won't play vocals lmao and certain notes in a song wont play either it's weird.